1. Field of the Invention
The invention relates to a fuel cell and a fuel cell stack. More specifically, the invention relates to a fuel cell including a metal porous body that serves as a gas passage layer, and a fuel cell stack including the fuel cell.
2. Description of Related Art
A polymer electrolyte fuel cell includes a membrane electrode assembly that is fowled by stacking a catalyst layer and a gas diffusion layer, in this order, on each side of an ion-permeable electrolyte membrane. The membrane electrode assembly is held between two gas passage layers, and the membrane electrode assembly held between the gas passage layers are held between two separators. In this way, a single cell is formed. Multiple cells are assembled together to form a fuel cell stack. Fuel gas that contains hydrogen is supplied to an anode (negative electrode), and an electrochemical reaction expressed by formula (1) below occurs, thereby producing protons from the fuel gas at the anode. The produced protons pass through the electrolyte membrane to reach a cathode (positive electrode). Oxidant gas that contains oxygen is supplied to the cathode (positive electrode), and an electrochemical reaction expressed by formula (2) below occurs, in which the oxygen reacts with protons from the anode (negative electrode), so that water is produced. The electrochemical reactions that occur at the electrolyte membrane-side surfaces of the paired electrodes are used to obtain electric energy from the electrodes.Anode reaction: H2→2H++2e−  (1)Cathode reaction: 2H++2e−+(½)O2→H2O  (2)
A fuel cell configured as described above is available, in which a metal porous body that is excellent in gas diffusion properties and electrical conductivity is used as the gas passage layer in order to supply fuel gas or oxidant gas to the electrode and collect electricity generated through the electrochemical reactions. Examples of the metal porous body include an expanded metal and a sintered metal foam. Conventional methods for producing the metal porous body include cutting and expanding a titanium sheet, a stainless steel sheet, or the like. For example, slits are made in a titanium sheet, a stainless steel sheet, or the like in a staggered manner and the sheet with slits is expanded by being stretched so that a mesh metal sheet, that is, an expanded metal is obtained.
The metal porous body obtained by cutting and expanding a metal sheet, in the above-described manner is cut into pieces having a given size in accordance with the external dimensions of the fuel cell. The obtained pieces are disposed on the respective sides of the membrane electrode assembly so that the fuel cell is faulted. The metal porous body may be cut into pieces by a laser cutter, or with the use of a die (Japanese Patent Application Publication No. 2010-80201 (JP-A-2010-80201)). With this method, a surface pressure or heat applied to the metal porous body may break an end surface of the metal porous body, which is a cut end surface, thereby causing clogging of pores. As shown in FIG. 7, an end surface 40 of a metal porous body functions as an inlet for gas that is supplied through a gas supply manifold portion 20. Therefore, clogging of the pores in an end surface portion of the metal porous body increases loss of gas introduction pressure, which may decrease, for example, the amount of gas that is supplied to the electrode, gas diffusivity, and electric power generation performance of the fuel cell.